Spark plugs that operate at high temperatures are commonly referred to as "hot plugs". Hot plugs have a tendency to pre-ignite the fuel mixture in the combustion zone. Pre-ignition of the fuel mixture in the combustion zone, also known as engine knock, reduces engine performance and in some instances, causes damage to the engine.
Another drawback to hot plugs is that deposits from fuel and lubricant additives which haven't burned away, are melted by the hot plug to form a glaze which coats the nose portion of the plug insulator. When the glaze on the nose portion of the plug insulator gets hot, the plug is shorted out and engine misfire occurs. Engine misfire is accompanied by fuel and power loss.
The main reason spark plugs operate at high temperatures is that the ceramic insulator surrounding the center electrode becomes hot due to combustion gas temperatures. Further, ceramic materials are normally poor conductors of heat.
Various devices for removing heat from the firing end of a spark plug have been used. One such heat removal device is a heat pipe having a vaporizable medium. Heat at the firing end of the spark plug is absorbed by the vaporizable medium causing such medium to convert to a vapor. The change of state of the vaporizable medium extracts heat from the firing end of the plug. The vaporized medium moves to a cooler part of the heat pipe where it condenses and releases heat through another change of state. The use of a heat pipe is an added expense and is a less than desirable heat removal method for use in the market place.
Another device for removing heat from the firing end of a spark plug is a thermally conductive filler composition introduced between the center electrode and the spark plug insulator. Some of the thermally conductive filler compositions in use include silver, cermet, talc, and silicon. Filler materials serve as a seal between the electrode and the insulator but only provide moderate improvement in thermal conductivity.
The present invention is directed to overcoming one or more of the problems as set forth above.